1. Field of the Invention
The present invention relates to the removal of hydrogen sulfide from gas streams with an aqueous washing solution and, more particularly, to a method for stabilizing washing solutions used in Stretford and similar hydrogen sulfide removal processes wherein absorbed hydrogen sulfide is converted to elemental sulfur.
2. Description of the Prior Art
With the increasing concern over atmospheric pollution and the concomitant increasingly strict enforcement of ever more stringent air pollution standards, greater responsibility is being placed on industry to produce pollution-free products in a non-polluting manner. One area of particular concern in the past, and still today, is the discharge or release of sulfur and its compounds, especially hydrogen sulfide (H.sub.2 S), into the atmosphere as a result of a number of industrial processes. These processes include, for example, petroleum refining, the roasting or smelting of various sulfide containing ores, the sweetening of sour natural gas, destructive distillation of coal and oil shale, gasification or liquefaction of coal, and the production and use of H.sub.2 S-containing geothermal steam and liquid for generating electricity and for other uses.
Several processes have been developed, and are in relatively common use, for removing H.sub.2 S from gas streams such as those generated or encountered in the industrial processes listed above. One such hydrogen sulfide removal process is the Stretford process, which employs an aqueous, alkaline washing solution to preferentially absorb and oxidize H.sub.2 S therein. Absorption is done with a water-soluble organic alkaline agent, such as anthraquinone disulphonic acid (ADA), with the hydrogen sulfide being oxidized to particles of elemental sulfur by a pentavalent vanadium compound such as sodium vanadate (NaVO.sub.3). Recovery of the sulfur is accomplished by flotation, using a stream of air which is injected into the process solution. This generates a frothy slurry containing the sulfur particles which rises to the top of the solution where it is skimmed off, with the sulfur therein being recovered therefrom by filtration or other liquid/solid separation techniques. In this process, the oxygen in the injected air also serves to reoxidize the reduced vanadate ions and thus regenerate the solution for reuse in the process.
There are a number of similar processes commercially available to perform such sulfur oxidation and recovery. One of these is the Unisulf process, which is described by Fenton et al. in U.S. Pat. No. 4,283,379, the disclosure of which is incorporated herein by reference in its entirety. In this process, the washing solution comprises a solubilized vanadium salt as the oxidizer, a non-quinone aromatic absorption compound, thiocyanate ions, and a water-soluble carboxylate complexing agent. Other processes are based on the use of other metallic oxidizers such as ferric iron and soluble arsenates and stannates.
A commonly used technique to remove the sulfur particles is by circulating the washing solution through a tank-like oxidizer vessel, through which air is bubbled to regenerate said washing solution and form said frothy slurry. With fresh, unused solutions, the elemental sulfur particles which, when formed, have an average diameter in the range between about 0.5 and 5.0 microns, agglomerate to form sulfur clumps of about 10 to about 150 microns in size. Particles of this size are readily buoyed up to the surface in the aforementioned froth and pass through a weir-like opening near the top of the vessel into a sulfur collection vessel. Here, the bubbles in the froth readily collapse, and the resultant liquid suspension or slurry can be easily pumped to a sulfur separation device such as rotary vacuum filter, filter press or centrifuge, from which, after washing to remove the entrained process solution, an extremely pure grade sulfur is obtained. Where a non-particulate form of sulfur is desired, the washed filter cake may be sent to an autoclave or other sulfur melter.
One problem which is sometimes encountered by the operators of many such processes is the introduction of hydrocarbons and/or one or more oxygenated organic contaminants such as alcohols, aldehydes and ketone into the incoming gas stream. Such an introduction may be caused, for example, by the incomplete or improper combustion of the sulfur contaminated waste stream introduced into a Claus process plant supplying the basic feed stock for this process. Another source may be the incomplete condensation and removal of such materials from shale retorting and coal gasification plants. Whatever the source, such materials have been found to cause a number of operating problems. Among these are an acceleration of the rate of formation of non-regenerable contaminants such as thiosulfates in the washing solution and the promotion of long-lasting, highly stable foams in the oxidizer vessel, which causes the formation of "sticky" sulfur particles and makes the subsequent sulfur separation and washing in the filter quite difficult.
When either of these situations occurs, the most usual practice is to treat it symptomatically. For example, when small bubbled foam, which is a very poor vehicle for transporting sulfur particles into the recovery system, and which also causes other operating problems, is present, it is often reduced, or prevented, by adding one or more anti-foaming agents to the solution. Past practices for treating high thiosulfate conditions frequently involve either purging a side stream and adding make up chemicals or, in extreme cases when the thiosulfate level gets too high (i.e., above about 600 g/l of washing solution), changing the entire system solution. However, environmental considerations are making such practices increasingly less attractive. Consequently, a number of processes involving treating the sidestream to destroy the thiosulfate, either by oxidation or reduction, have been developed. However, each of these approaches increases the overall costs of operating the process and adds yet other contaminants to the washing solution.
Instead of just purging a side stream to waste after the fact of excessive thiosulfate buildup, it would be better if there were an inexpensive, easily implemented, non-contaminating method by which hydrocarbon and oxygenated organic compounds at least partially responsible for accelerated thiosulfate buildup and excessive foaming could either be selectively removed from contaminated Stretford type washing solutions, along with some of the excessive thiosulfate, or, preferably, prevented from entering the system altogether. The present invention provides such a method.